This PPG will investigate the cellular, molecular and immunological properties of the cell surface inhibitory receptor programmed death-1 (PD-1) and how it functions during chronic viral infection. It is now well established that T cell dysfunction is a cardinal feature of many chronic viral infections and this is most strikingly seen during HIV infection. Recent studies have shown that the PD-1 inhibitory pathway plays a critical role in modulating this functional exhaustion of virus specific T cells. This has now been documented in several animal models such as murine LCMV infection and SIV infection of non-human primates and, more importantly, in humans during persistent infection with HIV, HBV and HCV. In addition to a role in regulating chronic viral infections, there is growing evidence that PD-1 plays an important role in tumors, autoimmunity and transplantation. However, despite PD-1's clear importance in maintenance of a healthy state little is known about PD-1 signaling and gene expression at the molecular level. With this in mind, the specific goals of this PPG are: 1) To elucidate the mechanisms of PD-1 function, signaling, and gene expression in healthy and HIV infected individuals and in model systems;and 2) To identify novel pathways, targets, and reagents that may be used to modify PD-1 expression and signaling in the treatment of immunological disease and viral infection. To achieve the above goals, we have assembled an outstanding team of investigators who will participate in the following four projects and three cores in this PPG: Project 1, PD-1 expression and HIV specific T cell dysfunction;Project 2, Regulation of PD-1 gene expression;Project 3, PD-1 signaling in T cells during chronic viral infection;Project 4, Targeting the PD-1 signaling pathway to rescue HIV T cell dysfunction. Taken together, these studies should provide insight into the mechanisms by which PD-1 regulates chronic viral infection and provide targets for therapeutic treatment of HIV infection. This molecular understanding of PD-1 function and regulation should also have implications for the treatment of tumors and autoimmunity, and for increasing the success of transplantation. PROJECT 1: PD-1 expression and HIV specific T cell dysfunction (Walker, B) PROJECT 1 DESCRIPTION (provided by applicant): T cell dysfunction upon ongoing antigen exposure is a cardinal feature of chronic infections and cancer in animals and humans, leading to T cell impairment and failure to eliminate virus or tumor cells. These defective T cell responses are thought to play a major role in HIV infection and progression to AIDS. Previous work performed at our center and by other investigators of this program application has shown in both HIV infection and animal models that the Programmed Death-1 (PD-1) pathway is a crucial mediator of T cell dysfunction, and that blocking this pathway can reinvigorate CD8 (CTL) and CD4 T cell responses. PD-1, its ligands PD-L1 and PD-L2, and the PD-L1 ligand CD80 participate in a network of pathways that modulate T cell function The goal of this project is to elucidate the role of PD-1 and its ligands in AIDS-related T cell dysfunction;successful achievement of these aims will help guide the design of new clinical interventions to reverse immune failure in HIV-infected subjects. In Aim 1, building on previous results correlating PD-1 expression by HIV-specific CD8 and CD4 T cells with disease progression, we will compare the expression and functional impact of PD-1 on T cells from subjects with acute infection, chronic infection, or spontaneous viral control (elite controllers). Using state-of-the-art imaging techniques from Core C, we will determine the role of PD-1 in modulating exocytosis of cytolytic granules by HIV-specific CTL. We will also investigate PD-1 expression and function in elite controllers who present a polymorphism in the PD-1 gene or in PD-1 regulatory elements, as identified in Projects 2 and 4. In Aim 2, we will determine qualitative differences between HIV-specific CTL expanded in the presence or absence of PD-1 blockade, with respect to immunodominance, cytokine production, and cytotoxicity. We will determine the roles of PD- 1 expression and TCR avidity in the response of HIV-specific CTL to PD-1 blockade. In Aim 3, we will determine the relative contributions of multiple pathways engaged by PD-1, PD-L1, and CD80 a key antiviral CTL function, namely the ability to suppress HIV replication in vitro. Starting from established methods to measure viral suppression, we will systematically knock down or block various PD-L1-mediated pathways to determine the role of each pathway in antiviral function. We will determine the relative roles of CTL proliferation, survival, cytokine secretion, and killing of infected targets in PD-L1 modulated antiviral activity. Relevance to public health: The immune system of HIV-infected people includes cells (T cells) that recognize HIV, but fail to block the virus and to prevent progression to AIDS. Our previous studies have shown that these T cells express a protein, called PD-1, which inhibits their function. We are studying the role of PD-1 in preventing proper T cell responses, and looking for ways to rescue the ability of T cells to control HIV.